1. Field of the Invention
The invention relates to a process for removing oxygen from a gas mixture containing hydrocarbon and oxygen, where the oxygen is at least partly converted into carbon dioxide and water in the presence of a catalyst.
2. Description of the Related Art
Gaseous hydrocarbons, for example ethylene, are standard basic chemicals of the chemical industry. Such gases can, for example, contain oxygen as impurity, which is a hindrance in the further use of the hydrocarbon gases and should be removed from the hydrocarbon gases. This can be carried out, for example, by oxidative purification in which the oxygen is reacted with the hydrocarbon to form carbon dioxide and water.
This catalytic, oxidative purification has already been practiced for a long time in the case of gas mixtures having relatively low oxygen contents of less than 5% by volume. Catalysts used for this purpose are palladium or platinum applied to supports composed of aluminum oxide, titanium oxide or silicon oxide. Thus, for example, U.S. Pat. No. 4,093,703 describes the oxidative removal of ethylene from gas mixtures containing up to 1.8% by volume of ethylene. To purify ethylene from crackers, US 2010/0048972 A1 teaches the use of ruthenium catalysts. The gases which are described therein and are to be purified contain oxygen in the ppm range. EP 0 499 087 A1 is concerned with the removal of traces of nitrogen oxide, carbon monoxide and hydrocarbon from oxygen-rich exhaust gases from gas turbines. GB 883,945 describes processes for purifying offgases from the oxidative conversion of ammonia into nitric acid, in which nitrogen oxides and oxygen are removed by addition of unsaturated compounds and sulfides. In Chemical Engineering and Processing 34, 1995, pages 469 to 478, Van de Beld describes the total oxidation of ethene and propane over Pd catalysts having Al2O3 supports.
In Environmental Engineering and Management Journal, 2003, Vol. 2, No. 4, pages 273 to 302, Rusu discusses the oxidation of ethene using Pd or Pt on TiO2, Al2O3 or SiO2 supports. In Catalysis Today, 122, (2007), pages 391 to 396, Hosseini recommends Pd and Au on TiO2 as a catalyst for the total oxidation of propene. Catalysts for purifying exhaust gases from diesel engines are known from EP 2 656 904 A1. The catalysts contain a catalytic coating comprising Pt, Pd and a carbon-storing compound such as zeolite and also a further coating comprising Pd and Au.
Catalytic, oxidative purification becomes problematical when the offgases consist predominantly of hydrocarbons and additionally have O2 concentrations of more than 5% by volume. This is because every catalyst requires a particular temperature above which it becomes effective, known as the light-off temperature, in order to achieve catalytic activity. Below this temperature, the catalyst has little effectiveness or is inactive and the reaction takes place only with unsatisfactorily small degrees of conversion. Above the light-off temperature, however, in the case of gas mixtures having relatively high proportions of hydrocarbons and oxygen it is possible for an explosive oxidation reaction and undesirable secondary reactions at the higher temperature level to occur, with the latter being the smaller problem.
Particularly when the gas mixture concerned has a high hydrocarbon concentration and/or high oxygen content and the catalyst requires a high light-off temperature, there is a potential risk of a runaway reaction. In the German patent application of Nov. 20, 2014 with the application number 102014223759.9, it is proposed that the gas mixture to be purified oxidatively be diluted with a gas mixture which has previously been purified. However, this means an additional outlay in terms of apparatus and an increase in the offgas volume. In the German patent application of Nov. 28, 2014 with the application number 102014224470.6, it is proposed that the light-off temperature of a palladium/gold catalyst be decreased by doping with an alkali metal. This means an increased outlay in catalyst production and thus makes the catalyst more expensive.